1. Field of the Invention
The invention relates to reinforcement for concrete structures. In particular, the invention relates to a T-headed stirrup which is useful for providing high resistance to punching shear in concrete walls and in restraining buckling of reinforcing bars under high compression.
2. Description of the Related Art
Reinforced concrete is commonly used in construction. Metal reinforcements are used to improve the strength of concrete structures subjected to severe loads and forces, particularly tensional and shear forces. Steel reinforcing members are often placed both longitudinally and transversely within such structures.
Reinforced concrete is desirable in ocean and arctic construction, such as in a shaft or caisson designed to resist the impact of boats, movement of icebergs or sea ice, explosion, etc. In walls, for example, the primary reinforcement to resist bending (flexure) is placed near each face of the wall, i.e., near both the interior and exterior face. This reinforcement consists of either individual bars or bundles of bars on relatively close spacing, which run in two orthogonal directions, usually horizontally and vertically. To resist punching shear, transverse reinforcement (such as a stirrup) is also installed on close spacing, locking around the primary reinforcement.
According to a definition in "Cement and Concrete Terminology" in the ACI Manual of Concrete Practice (Part 2--1983, American Concrete Institute, Detroit, Mich.), a stirrup is a reinforcement used to resist shear and diagonal tension stresses in a structural member; it is typically a steel bar bent into a U or box shape and installed perpendicular to or at an angle to the longitudinal reinforcement, and is properly anchored.
Further discussion of bar stirrups can be found in Reinforced Concrete Fundamentals, Fourth Edition, by Phil M. Ferguson (New York: John Wiley & Sons, 1979). As explained therein, a stirrup in a reinforced concrete beam can be a simple bar which is well anchored in the compression zone of the beam and is bent around the longitudinal tension bars, so that it functions as vertical reinforcement. As shear stress is applied, the stirrup goes into tension and thus controls and limits the progress of any cracking of the concrete. Until the stirrup either pulls out or yields, the beam will not experience shear failure.
In a horizontal concrete beam which contains both longitudinal and transverse reinforcement, stirrups provide "vertical" reinforcement. In certain structures such as sea walls designed to resist lateral forces, the stirrups are actually horizontal, not vertical. For uniformity of description in this patent, stirrups will be said to provide "perpendicular" reinforcement since they tend to restrain movement perpendicular to the plane formed by the longitudinal and transverse reinforcement.
With conventional bar stirrups, the amount of perpendicular reinforcement is physically limited by geometry and the allowable bend radii. Failure of conventional stirrups is usually by crushing under the bend or pull out of the bar ends, well below yield, thus making the stirrups inefficient. Conventional stirrups also require extensive labor to place.
Stirrups consisting of planar plates welded onto the ends of shafts are an improvement over simple bars because the plates can be anchored behind the primary reinforcement. However, failure of these occurs when the concrete cover, outside the reinforcement, spalls and the primary reinforcing bars deflect sideways. This type of stirrup is also difficult to place.
An improvement over simple bar stirrups or similar mechanical anchorages can be found in the prior art. U.S. Pat. No. 2,157,271 to E. E. Schmeller discloses a concrete wall structure, where the interior structure of the wall section is reinforced by the use of wires that are crossed to form the usual mesh, and are used in conjunction with cast metal tie bars extending transversely of the section and fashioned with retaining heads imbedded in the face walls.
Another improvement has been made over stirrups with headed plates and bars such as described in Schmeller. Instead of a planar endplate, a shaft having T-heads at opposing ends and substantially flat surfaces was devised. This flat shape made it practical to insert the T-headed stirrup between two closely-spaced reinforcing bars, then turn the stirrup ninety degrees so that the T-head contacted the reinforcing bar.
A flat T-headed stirrup provided satisfactory performance by inhibiting transverse buckling of the longitudinal reinforcing bars in concrete. However, once the concrete surface covering the reinforcing bars had spalled, lateral buckling and movement of the reinforcing bars was essentially unrestrained.